If you ask him, he will feel like he’s not doing his personal grooming right. He might take offense at that. It’s not nice to be told you aren’t clean. Some guys may not mind, and others would.
Second, he might think, “grow up.” “Grooming” pubic hair is something that has been popularized by porn. People nowadays think that’s how it’s supposed to be, and give this “hair in the mouth” crap as an excuse for it. If you’re giving a blow job, there’s no reason to get a hair in your mouth unless you’re hoovering the wrong area. This is an issue of being like a porn actor. Things were not like this only thirty years ago, and no one then was complaining. Women in porn need to shave so the camera can see more.
Guys are probably more likely to get hair in their mouths and, I suppose, might request a trim. That seems pretty weird to me. If it doesn’t look the way it would naturally, it can make a woman look like a prepubescent girl if she goes bald, which is very freaky.
Anyway, if it’s that important to you, then you are right to worry. He could take offense, and rightly so. But I don’t see what you can do other than make a case for it, suggesting you’d go down on him a lot more if he were more closely shaved. It would make his cock seem longer, which would excite you no end.